The management of the subjective loudness of broadcast audio signals, both in radio and television, has been important since the early days of broadcasting. While the viewer or listener invariably has control over the overall volume level of the content they are observing, it is important for the broadcaster to manage the variations in perceived loudness over the course of a programming schedule to meet the requirements of intelligibility, comfort, impact and artistic expression. These requirements can come into conflict. For example, classical music may require a high dynamic range in order that the listener may appreciate the contrasts between solo instruments playing quiet passages and the full orchestra, complete with percussion, reaching a climax in the piece. However, the limitations of a particular transmission channel and listening environment often require a lower dynamic range, to prevent either the quiet passages being inaudible or the loud passages being distorted. The technique of compression (not to be confused with bandwidth or bit rate reduction) has long been used to adapt the dynamic range to the conditions of the channel or listening environment. In the move from analogue to digital broadcasting, and with the increasing quality of home amplification and loudspeaker systems, compression has become less desirable.
Another significant area of conflict in the management of loudness concerns the relative perceived loudness of programme content and commercials or promotional content. In a film or television drama, the director achieves significant artistic impact through the dramatic use of contrasts between quiet and loud scenes. In order to accommodate both, the average loudness of such a programme may be quite low. Conversely, in a television commercial, the director's goal is to maximize impact over the short duration of the commercial, a goal which is often achieved by making the commercial loud. In the days of analogue broadcasting, when compression was applied quite strongly, the subjective discrepancy between the loudness of programmes and commercials was not a significant problem. In digital broadcasting, however, this discrepancy has become a significant source of annoyance for viewers and listeners.
Two developments have helped to overcome this problem. The first is the emergence and standardization of reliable methods of measuring, controlling and logging subjective loudness. These methods are described in ITU Standard BS-1770, “Algorithms to measure audio programme loudness and true-peak audio level” and EBU Recommendation R128, “Loudness normalisation and permitted maximum level of audio signals”. The second development is the political will of broadcast industry regulators to introduce rules, and in some cases legislation, to control the relative loudness of programmes and commercials, for example in the CALM (Commercial Advertisement Loudness Mitigation) Act introduced by the FCC in the U.S. Such rules impose limits on the relative loudness of commercials and the programmes they accompany.
These developments have led to a significant improvement in the overall comfort of viewers and listeners as concerns audio loudness. However, the inventors have recognized that there remains the potential for annoyance at the junctions between programmes and commercials, even when legal and contractual requirements for average loudness have been met. A typical scenario is when a fast-action drama ends with a relatively silent scene. An abrupt transition from such a scene to a loud commercial can cause considerable annoyance to the observer. An equivalent transition within a programme is not so much of a problem because it will have been introduced for artistic effect.
It is the object of this invention to provide, for both live and pre-recorded broadcasting, a method and apparatus for mitigating the effect of abrupt loudness transitions at junctions between programmes and commercials or promotional content.